1. Field of the Invention
The present invention relates to a process for converting purified, partially purified or crude taxane mixtures into a protected precursor of 10-deacetylbaccatin III or into 10-deacetylbaccatin III. Specifically, the present invention relates to a three-step process whereby in the first step the 2'-OH and the 7-OH groups of 10-deacetyl taxol A, B or C are protected using a suitable protecting group. Cleavage of the side chain located at the C-13 position occurs during the second step of the process resulting in a protected precursor of 10-deacetylbaccatin III. Deprotection occurs during the third step resulting in 10-deacetylbaccatin III.
2. Description of the State of Art
Between the years 1958 and 1980, extracts of over 35,000 plant species were tested for anticancer activity as part of an NCI-sponsored program. Chemists Monroe E. Wall and M. C. Wani first isolated a crude extract concentrate from yew tree (Taxus brevifolia) bark and wood samples in 1963. Initial screening showed the extract to be a potential anticancer agent, being very active against an unusually wide range of rodent cancers. Isolation of the active agent in the crude extract took several years due to the very low concentrations of the agent present in the plants. The active agent was identified, the structure determined and the compound was named taxol (I), in 1971. ##STR3## Despite taxol's excellent activity in model tumor systems, clinical trails were delayed owing to short supplies of the drug and formulation problems related to the drug's low water solubility. However, great interest in the drug was rekindled when it was discovered in 1979 by Susan B. Horwitz and co-workers that a unique mechanism for taxol's antitumor activity involved cell microtubules. See, Nature 277:665-667 (1979). Microtubules play a key role in mitosis, maintenance of cell shape, cell motility, and intracellular transport. They are self-assembling and self-disassembling structures that are in dynamic equilibrium with tubulin dimers, the protein subunits of which they are composed. A substance that interferes with microtubules can disrupt cell growth and function.
The 1979 study by S. Horwitz et al., reported that the binding of taxol to tubulin acts to stabilize cell microtubules and to prevent their depolymerization. Thus, taxol increases the time required for cell division which in turn inhibits tumor activity. Discovery of this unique mechanism, by which taxol disrupts the proliferation of cancerous cells, intensified research interest in the drug, and the National Cancer Institute (NCI) began a concerted effort to obtain taxol for clinical trials. In ongoing clinical trials, taxol has shown promising results in fighting advanced cases of ovarian, breast, and other cancers. Recently, taxol was approved by the Food and Drug Administration for the treatment of refractory ovarian cancer; however, taxol is extracted in limited quantities from a natural vegetation that is in short supply.
There have been some methods developed for increasing the supply of taxol. For example, tissues of Taxus brevifolia have been successfully cultured to produce taxol, related alkaloids, and alkaloid precursors, as disclosed in U.S. Pat. No. 5,019,504 issued to Christen et al. Turning toward an alternative route, Holton et al., JACS 110:6558 (1988) proposed a synthetic route, directed to the synthesis of the tetracyclic taxane nucleus from commodity chemicals. Despite the progress made in this approach, the final total synthesis of taxol is, nevertheless, likely to be a multi-step, tedious, and costly process.
An alternate approach to the total synthesis of taxol has been a partial synthesis or semi-synthetic route, involving the use of related alkaloid precursors, collectively referred to as taxanes. U.S. Pat. No. 4,924,011 issued to Denis et al., discloses a process whereby taxol is prepared from a derivative of 10-deacetylbaccatin III or baccatin lII. The U.S. Pat. No. 4,857,653 issued to Colin et al. discloses processes for the preparation of taxol and 10-deacetyl taxol from baccatin III and 10-deacetylbaccatin III, respectively. Carver et al., in his U.S. Pat. No. 5,202,448, discloses a method whereby partially purified mixtures of taxanes containing taxol and cephalomannine are converted into baccatin III.
The above U.S. patents and technical paper by Holton each disclose a process whereby the supply of taxol may be increased; however, the amount of taxol actually produced by way of tissue cultures as disclosed by Christen et al. is minute, and although 10-deacetyl taxol is known to promote in vitro the polymerization of tubulin and to inhibit, at the same time, the depolymerization of microtubules, the only taxane currently approved for use as a chemotherapeutic agent is taxol. Consequently, other naturally occurring taxanes that are similar in structure such as 10-deacetyl taxol are useful and valuable only if they can be ultimately converted to taxol. The semi-synthetic route disclosed in the Denis et al. patent is unfortunately limited by the precursors required for the starting material. The Carver et al. patent addresses this need by disclosing a process whereby baccatin III, a precursor for the semi-synthetic route may be obtained. However, the use of sodium borohydride as a reducing salt resulted in an undesired epimerization at the C-7 position, decreasing the yield of baccatin III. To counteract the epimerization event at the C-7 position, Carver et at. discloses the use of Lewis acids. The heavy metal halides used are extremely toxic and it would be undesirable to introduce this type of toxic into a process for a pharmaceutical drug.
There is still a need, therefore, for a process to convert other naturally occurring taxanes to taxol or to taxol precursors that can be easily converted into taxol.